From former years, a polyethylene terephthalate film (PET film) is used widely as a base film for magnetic tapes. In recent years, magnetic tapes are being increasingly required to enable high-density recording for downsizing and for achieving high-quality images. Further, it is also required to decrease the thickness of magnetic tapes, as is typically required of 8-mm video tapes. For these reasons, the magnetic layer-side surface of a magnetic tape is required to be smoothest possible, and the thickness of a magnetic tape is required to be smallest possible.
Due to this, a base film is also required to have a flatter surface and a smaller thickness. However, a PET film used as a base film of conventional home VTR magnetic tapes has a rough surface, and many of PET films fails to satisfy the above-required properties for practical use.
For applying a PET film to a base film for a magnetic tape which permits high-density recording, it is basically required to extremely decrease the surface roughness. However, when the surface roughness is decreased, generally, the lubricity between film surfaces deteriorates, and further, air entrapped between film surfaces is hardly or poorly released, which makes it very difficult to take up the film on a roll. This difficulty also increases as the PET film thickness decreases.
Moreover, as the film thickness decreases, the film is required to have a higher Young's modulus in order to retain the film strength.
A PET film having a high Young's modulus can be obtained by increasing the stretch ratio. However, a film treated at a high stretch ratio generally has a high shrinkage, and a magnetic tape formed therefrom is poor in dimensional stability.
Further, in producing a magnetic tape, a magnetic layer is formed on the surface of a PET film treated at a high stretch ratio, and its surface is treated for smoothing it. Then, the magnetic tape is taken up in the form of a roll, and the magnetic layer of the rolled magnetic tape is thermoset. In the heat treatment for this thermosetting, the smoothly finished magnetic surface and the base film surface are frictionally and tightly contacted to each other due to heat shrinkage. Therefore, a phenomenon that the finished smooth magnetic surface is roughened (phenomenon of imprinting film surface roughness) is amplified, and the electromagnetic characteristics are deteriorated.
For overcoming the above defects of a PET film, Japanese Laid-open Patent Publication No. 62-164,733 (164,733/1987) proposes a biaxially oriented PET film for magnetic recording, which is formed from a polyester containing 0.1 to 0.5% by weight of inert solid particles having an average particle diameter of 0.03 to 0.3 .mu.m as small-sized particles, and 0.001 to 0.05% by weight of inert solid particles having an average particle diameter of 0.2 to 0.8 .mu.m as large-sized particles, a size difference between the large-sized particles and the small-sized particles being at least 0.15 .mu.m in average particle diameter, and which has a surface roughness Ra of 0.003 to 0.012 .mu.m, a Young's modulus of at least 650 kg/mm.sup.2 in the longitudinal (machine) direction, and a heat shrinkage of 0.06% or less when heat treated under no load at 70.degree. C. for 1 hour.
On the other hand, polyethylene-2,6-naphthalenedicarboxylate is available as a polyester which is excellent in heat resistance and strength, and it inherently has properties for giving an excellent base film for a magnetic recording medium. And, it has been proposed to use the above polymer for forming a base film for a magnetic recording medium, as will be described below.
For example, Japanese Laid-open Patent Publication No. 58-215,722 (215,722/1983) proposes a polyester film for a magnetic recording medium, whose average refractive index n and plane orientation degree .DELTA.P satisfy the following expressions [1] and [2] and whose average protrusion height on the film surface is 0.015 .mu.m or less. EQU n.gtoreq.1.600 [1] EQU .DELTA.P.ltoreq.1.43.multidot. n-2.128 [2]
In the above proposal, the polyester includes a polyester produced from 2,6-naphthalenedicarboxylic acid and ethylene glycol.
Japanese Laid-open Patent Publication No. 63-60,732 (60,732988) proposes a polyethylene-2,6-naphthalate film containing 0.01 to 10% by weight of inert fine particles having an average particle diameter of 0.1 to 10 .mu.m, in which the 1/2 crystallization time at 200.degree. C. is 6.0 minutes or less; the plane orientation degree .DELTA.P and an average refractive index n satisfy the following expressions (1) and (2), EQU .DELTA.P.ltoreq.1.43.multidot. n-2.136 (1) EQU 1.6650.ltoreq. n.ltoreq.1.6750 (2);
the number A (piecesm.sup.2) of concavo-convex units, each of which is formed of a protrusion and a dent being present around the protrusion as a core and having a major diameter of at least 0.2 .mu.m, per 1 mm.sup.2 of the film surface area satisfies the following expression (3), EQU 0&lt;A.ltoreq.15,000 (3);
and the variance of thickness in the film length and transverse directions is 8% or less, the film being a polyethylene-2,6-naphthalate film which has a flat surface and lubricity and is excellent in thickness uniformity. It is described that the above film is excellent as a base film for a magnetic recording medium.
Further, Japanese Laid-open Patent Publication No. 62-143,938 (143,938/1987) proposes a polyethylene-2,6-naphthalate film for a high-density magnetic recording medium which is formed of a biaxially oriented film composed mainly of a polyethylene-2,6-naphthalate film, the film satisfying the following:
(a) the tensile stress in every direction in its plane at 5% elongation is at least 13 kg/mm.sup.2 and less than 21 kg/mm.sup.2,
(b) the Young's modulus in every direction in its plane is at least 500 kg/mm.sup.2 and less than 800 kg/mm.sup.2,
(c) the heat shrinkage in every direction in its plane is less than 2.5%,
(d) the central line average roughness Ra on the film surface is 0.012 .mu.m or less, and
(e) the number of coarse protrusions having a height of at least 1.08 .mu.m is 10 pieces/25 cm.sup.2 or less, the in-plane deviations of the above (a), (b) and (c) values being 20% or less.